Reactors used in the chemical, petroleum refining and other industries for passing liquids or mixed-phase liquid/vapor mixtures over packed beds of particular solids are employed for a variety of different processes. Typical of such processes in the petroleum refining industry are catalytic dewaxing, hydrotreating, hydrodesulfurisation, hydrofinishing and hydrocracking. In these processes liquid phase is typically mixed with a gas or vapor phase and the mixture passed over a particulate catalyst maintained in a packed bed in a downflow reactor. Because chemical reactions take place which themselves may produce additional components in the vapor phase, for example, hydrogen sulfide and ammonia during hydrotreating processes, and because such reactions may consume some of the vapor phrase reactants, it is frequently necessary to add additional vaporous reactants e.g. hydrogen at various points along the path of the reactants. Other reactions may use heat exchange media, e.g., hydrogen quench, which are added or withdrawn at different points in the unit. To do this, the contact solid is conventionally arrayed in superimposed beds with a distributor plate above each bed in the sequence to ensure good distribution of the reactant phases at the top of the bed so that flow is uniform across the beds, at least at the top of the bed. By ensuring good reactant distribution, the bed is used most effectively and efficiently and the desired reactions will take place in the most predictable manner with a reduced likelihood of undesirable exotherms or other problem conditions.
Many different types of distribution plate are known. Some are simple and comprise little more than a pierced or slotted plate. Others have various forms of weirs or other devices for promoting the desired uniformity of reactant flow, achieving good liquid/vapor contact. For example, reference is made to U.S. Pat. No. 4,126,539 which shows a distributor plate for use in a catalytic hydrodesulfurisation (CHD) reactor.
One type of system involves an inlet deflector cone cooperating with a splash plate and liquid distributor trough to pass liquid into the reactor to two distributor trays which facilitate the uniform spreading of liquid over the upper face of the catalyst bed. The distributor trays contain a series of spaced risers which have dual functions. They permit vapor to pass down through the tray and they also serve as liquid downflow conduits, the liquid passing through weir slots in the sides of the risers. The nature of liquid flow through weirs, however, makes this type of design very sensitive to tray unevenness introduced during fabrication or installation.
Another example of a distributor is the mixed-phase flow distributor for packed beds of U.S. Pat. No. 3,524,731, which was intended primarily to accommodate wide variations in liquid feed rate. The liquid flow is normally through liquid downpipes but at very high liquid rates, some liquid overflows into the vapor chimneys through triangular weirs. However, during normal operation the chimneys do not carry liquid and hence do not contribute to the number of liquid streams entering the bed. Also, during periods when they carry liquid there would be a a great variation in the liquid flow through the chimneys compared with that through the tubes.
U.S. Pat. No. 3,353,924 shows a somewhat different approach: flow into the liquid tubes is still through a pair of notched weirs and the disadvantages mentioned above are applicable wth respect to this system as well. There is no liquid flow in the vapor chimneys, and the number of uniformly spaced liquid streams which can be placed on the tray is therefore limited.
A system of this kind is shown, for example, in U.S. Pat. No. 4,126,539. Liquid flow is through the vapor downcomers only, by a combination of hole and weir flow. The tray area between the downcomers is not used for liquid distribution, and the use of weir flow makes the distribution pattern vulnerable to variations in tray level.
Other approaches appear in U.S. Pat. Nos. 4,126,540 and 4,140,625 where liquid flow is through holes in downcomers only. There is no attempt to make use of the tray area between downcomers and the size of the downcomers, coupled with the need to maintain tray mechanical integrity, prevents maximization of the number of liquid streams entering the catalyst bed.
Liquid distribution is also of concern in other environments. For example, in U.S. Pat. No. 2,924,441 the disclosure is related to the design of a liquid distributor for gas/liquid phases such as in gas absorption of distillation in a packed tower. The distributor described makes no attempt to address the special need for good initial liquid distribution found in concurrent downflow catalytic reactors.
Another form of distributor is shown in U.S. Pat. No. 3,541,000. The system employs a plate fitted with liquid downcomers which maintain the desired level of liquid above the plate before overflow into each downcomer which also has to allow for the vapor to pass into the bed beneath. This system has two disadvantages. First, the configuration of the top of the downcomers permits considerable variations in the liquid flow rate across the plate unless it is fabricated and installed in a completely horizontal position. The liquid flow rate into the downcomer increases exponentially with the liquid height above the lower edge of the weir and so, if the plate is not horizontal, the greater height of the liquid at one edge of the plate will give a greatly increased liquid flow on the low side of the plate at the expense of the high side. The use of the downcomers for liquid and vapor flow exacerbates this problem since vapor will not flow down through the liquid to a submerged aperture. Thus, if the weirs on the low side of the plate become submerged, not only will the liquid flow increase greatly but vapor flow may be cut off completely. Thus, the desired reactions may be almost completely precluded on at least one side of the reactor bed.